Apple provides a direct download for the El Capitan installer as a disk image (.dmg) file. Unlike modern versions, this isn't found in the main Mac App Store search.
Once the download finishes, the installer may launch automatically. if you want to create a USB drive. Quit the installer first. download mac os el capitan installer
You can download on any Mac, but to El Capitan, your Mac must be one of these: Apple provides a direct download for the El
Here is the safe, official method to get the installer and how to create a bootable drive. if you want to create a USB drive
Once you've downloaded the macOS El Capitan installer, you can install it on your Mac:
To perform a clean install, you should create a bootable USB drive (at least 16GB). El Capitan full installer - Apple Support Community
